1、Designation: E471 96 (Reapproved 2011)Standard Test Method forObtaining Char Density Profile of Ablative Materials byMachining and Weighing1This standard is issued under the fixed designation E471; the number immediately following the designation indicates the year oforiginal adoption or, in the cas
2、e of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONThe ablation characteristics of charring materials must be well known in order to design th
3、e materialfor a specific set of environmental conditions. The char density profile and the environmentalconditions under which it was formed can provide useful information about the ablation performance.A method of obtaining the char density profile from a charred sample of material is described in
4、thefollowing sections. Some chars are very friable and are easily broken before they can be measured.Other chars are relatively strong and can be handled with ease. The type of char density profilemeasurement described in this method is applicable if the char is strong enough to be machinedwithout b
5、reaking.1. Scope1.1 This test method covers the determination of the chardensity profile of a charred ablator that can be used with thefollowing limitations:1.1.1 The local surface imperfections must be removed, andthe char must be able to be machined off in a plane parallel tothe char-virgin materi
6、al interface before the density profilescan be determined.1.1.2 The char must be strong enough to withstand themachining and handling techniques employed.1.1.3 The material should have orderly density variations.The total thickness of the char and degradation zone must belarger than the machining th
7、icknesses required.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.2.1 ExceptionCertain inch-pound equivalent units areincluded in parentheses for information only.1.3 This standard does not purport to address all of thesa
8、fety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Summary of Test Method2.1 Density variation throughout a charred ablat
9、or materialis determined by successively measuring, machining, andweighing a sample of known size to obtain the density of thematerial removed by machining.3. Apparatus3.1 The apparatus required for this method includes alaboratory balance capable of measuring to the nearest 0.0001g, and a machining
10、 technique capable of removing material inincrements as small as 0.025 mm (0.001 in.). For example, flatspecimens can be machined with a surface grinder using amedium fine grit ceramic grinding wheel of a soft gradedressed to the proper contour. Cylindrical specimens can bemounted in a lathe and the
11、 char can be removed with a sharpcarbide or diamond tip tool.4. Sampling4.1 The charred sample selected for machining and weigh-ing should be taken from a representative section of the ablatedspecimen where the environmental conditions are well known,and where the surface is parallel to the char-vir
12、gin materialinterface. Where large sections are available, this condition isusually met. For small samples which have been exposed to1This test method is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommitt
13、ee E21.08 on Thermal Protection.Current edition approved Jan. 1, 2011. Published April 2011. Originallyapproved in 1973. Last previous edition approved in 2002 as E471 96 (2002).DOI: 10.1520/E0471-96R11.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-29
14、59, United States.varying environmental conditions along the length of thesample, the sample size will be smaller.5. Test Specimens5.1 A typical specimen size obtained from a channel, pipeflow, or rocket motor section may be 12.7 by 12.7 mm (0.5 by0.5 in.), or 12.7 mm (0.5 in.) in diameter. The samp
15、le thicknessis determined by the available thickness of material. Smaller orlarger samples can be used depending upon the accuracy,weighing apparatus, and specimen size. Larger samples reduceedge effects. Excess virgin material should be eliminated, ifpossible.5.2 The specimen is rough-cut out of th
16、e ablated section,and then machined so as to make the sides perpendicular andthe front surface parallel to the char-virgin interface as shownin Fig. 1.5.3 For simplicity and ease of handling, the base of thesample (the backface or side opposite the charred surface) maybe cemented to a thin piece of
17、steel. This permits the sample tobe handled with tweezers and facilitates the mounting of thesample on a magnetic chuck or other mounting jig formachining the surface from a fixed reference plane.6. Procedure6.1 Sample Preparation:6.1.1 Take the dimensions of the specimen with microme-ters or other
18、suitable measuring devices to the nearest 0.025mm (0.001 in.), and record. Weigh the specimen on a balanceto the nearest 0.0001 g and record the weight. If the sample hasnot been prepared with parallel sides, take additional measure-ments to determine the surface area being removed. Section 6.4has a
19、dditional comments on moisture content and chemicalreactions of some chars with atmospheric constituents.6.1.2 Take care to prevent breaking off any of the charduring the measuring or weighing operations.6.2 Machining:6.2.1 If the char-virgin material interface is not parallel tothe front surface, t
20、hen machine the latter parallel to theinterface.6.2.2 It is preferable to have a grinding machine, or lathe setup specifically for this machining operation so as to leave thesettings untouched between weighings. For a surface grindingmachine, advance the grinding wheel dressed flat to 60.0127mm (0.0
21、005 in.) in 0.025-mm (0.001-in.) increments until asuitable amount has been removed. This is usually about 0.254mm (0.010 in.). After the surface has been cleaned up (whenthe machining process has resulted in a continuous surface allover the specimen) allow the specimen to stabilize at theselected r
22、elative humidity and temperature conditions beforemeasuring and weighing. For consistency, remove the machingdust after each machining operation with about 34 kPa (5 psi)air through a nozzle held about 25 mm (1 in.) from themachined surface.6.3 Dimensional Measurements:6.3.1 The increment of materia
23、l removed between weigh-ings is usually 0.254 mm (0.010 in.) depending upon the totalchar thickness and the uniformity of the charred specimen. Ifthe char is relatively uniform in density down to the char-virginmaterial interface, increase the machining increments. If thechar-virgin material interfa
24、ce is sharply defined, take smallermachining increments. Exercise extreme care when making thedimensional measurements with a micrometer to avoid crush-ing the char.6.4 Weight Measurements:6.4.1 Maintain the specimen at constant humidity condi-tions (ideally dry) during the machining, measuring, and
25、weighing operation. A freshly charred ablator has a chemicallyactive surface like that of activated charcoal. Previous experi-ence indicates equilibrating the specimen to 50 % relativehumidity and 20C (70F) dry should yield good results.2Make a separate measurement on the char to determine howmuch m
26、oisture is contained at this relative humidity.6.4.2 Some chars have chemically active species which areunstable in atmospheric conditions. Special handling tech-niques would be required for these chars and this is beyond thescope of this test method.6.4.3 Repeatedly machine, measure, and weigh the
27、speci-men until the char and the char-virgin material interface hasbeen removed, and the density approaches the virgin materialdensity.7. Calculation7.1 Determine the average density of the material removedusing the weight and dimensional changes as follows:2Liston, E. M.,“Experimental and Analytica
28、l Studies of Radiation-Only Pyroly-sis of Model Char-Forming Polymers”, Contract NAS-7-341, SRI Report No. 19,Final Report 1968.FIG. 1 Typical Specimen for Measuring Char Density ProfileE471 96 (2011)2r5W12 W2!/AH12 H2! (1)where:r = average density, kg/m3,W1= weight before machining, kg,W2= weight a
29、fter machining, kg,A = surface area of specimen face, m2,H1= height of specimen before machining, m, andH2= height of specimen after machining, m.8. Report8.1 Report the density calculated for each machining step atthe point mid-way between the height measurement for eachincrement removed. Report th
30、e density on a moisture-free basisusing the measured relative humidity and temperature condi-tions taken when weighing the specimen. Report the averagerelative humidity and temperature conditions. If the moisturecontent of the char has not been determined as a function ofrelative humidity and temper
31、ature then report these conditionsand the time of equilibration at these conditions beforeweighing.9. Precision and Bias9.1 The precision resulting from the machining and weigh-ing technique described here is inversely proportional to theheight removed between weighings and the accuracy of themeasur
32、ing device used to determine the dimensions betweenweighings.An error of less than 3 % can be achieved where theheight removed is on the order of 0.254 mm (0.010 in.) and thesample is not smaller than 12.7 mm (0.5 in.) in length andwidth, or diameter. The precision is also dependent upon themoisture
33、 content of the char and the variation in relativehumidity and temperature during machining and weighing.39.2 The bias of this method is dependent upon the friabilityof the sample, the density gradient, the size of the specimen,the uniformity of the char, the moisture content of the char, andthe var
34、iation in relative humidity.10. Keywords10.1 ablation; char density; density profileASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the
35、 validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved or withdrawn. Your
36、 comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel that your comments
37、have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints (single or multi
38、ple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org). Permission rights to photocopy the standard may also be secured from the ASTM website (www.astm.org/COPYRIGHT/).3Hiester, N. K., and Clark, C. F., “Comparative Evaluation ofAblating Materialsin Arc Plasma Jets,” NASA CR-1207, 1968.E471 96 (2011)3
